Typical baseband composite audio stereo signals include a main signal formed from the sum of the left and right channel signals (L+R), a pilot signal having a frequency .omega..sub.p which is greater than the maximum frequency of the (L+R) signal, and a difference signal (L-R) which is the difference between the left and right channel signals. The difference signal is in the form of a double sideband, amplitude modulated suppressed carrier centered at a frequency of twice .omega..sub.p. The pilot signal is essential for demodulating the suppressed carrier to extract the (L-R) audio information.
The demodulated (L-R) signal will nominally include a component c-orresponding to the pilot signal, and the maximum signal frequency of the (L+R) signal is nominally relatively close to the pilot signal frequency. The pilot signal must be removed from both the (L-R) and the (L+R) signals to preclude it from introducing undesirable high frequency tones in the reproduced sound signals. The pilot signal may be removed by suitable low-pass filters, however, to do so requires filters with relatively sharp cutoff characteristics. Such filters are complex and, therefore, undesirably expensive for application in consumer products.
Minoru Ogita in U.S. Pat. No. 4,404,430 entitled "FM Stereo Demodulation Circuit With Voltage Dividing And Multiplexing Techniques", disclosed alternate pilot signal cancelling apparatus wherein a synthesized pilot signal is generated and subtracted from the composite audio signal. In the Ogita apparatus the pilot signal is cancelled at a point in the circuitry subsequent detection of the phase of the pilot signal for generating a clock signal to demodulate the suppressed carrier.
At the present state of the art of integrated circuit fabrication it is practical to implement substantially the entirety of audio processing circuitry on a single silicon integrated circuit die. To do so, however, it is desirable to constrain certain signal parameters within limits to reduce power consumption or the actual number of devices on the die. The former limitation is important for analog integrated circuit designs and the latter is important for digital integrated circuit designs. The constraints may be satisfied for both cases if the peak amplitude of the composite signal is reduced. This reduction may be effected by cancelling the pilot signal prior to the bulk of the signal processing apparatus including phase detection of the pilot signal. While it may seem contradictory to be able to accomplish detection of the phase of the pilot signal after the pilot signal is cancelled, it will be demonstrated in the following description of the present invention that such is possible.